Extensive psychophysical experiments have established that the sense of respiratory effort bears a power function relationship to the force generated by respiratory muscles and the frequency and duration of their contraction, and an inverse relationship to respiratory muscle strength. Using this information an approach to the assessment of dyspnea is developed which has several components. First, the response to exercise, in terms of the metabolic and gas exchange demands is established. Second, the ventilatory responses are analyzed in terms of total ventilation, the pattern of breathing (tidal volume and breathing frequency), and the timing of breathing (inspiratory and expiratory times). Third, the exercise measurements are considered in relation to the elastic and resistive components of the respiratory system that impede volume and flow, as reflected in measurements of static lung volume and the flow:volume characteristic. Finally, measurement of the maximum inspiratory pressure against an occluded airway reflects respiratory muscle strength. Dyspnea, the sensation of increased respiratory effort, is usually due to a combination of abnormalities--increased metabolic demand, increased ventilatory response, increased impedance to breathing, and reduced respiratory muscle power. Management of abnormality in each of these components is distinctly separate.
